/Engine/Source/Runtime/Online/HTTP/Public/HttpRequestAdapter.h
Appends to the value already set in the header. If there is already content in that header, a comma delimiter is used. If the header is as of yet unset, the result is the same as calling SetHeader Content-Length is the only header set for you. Also see:SetHeader()
virtual void AppendToHeader ( const [FString](API\Runtime\Core\Containers\FString) & HeaderName, const [FString](API\Runtime\Core\Containers\FString) & AdditionalHeaderValue )